Why Scientists Keep Changing the Forecast for a La Niña Winter

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Why Scientists Keep Changing the Forecast for a La Niña Winter

VALERY HACHE/AFP/Getty Images

Climate predictions—kind of like romantic comedies—are full of will they/won't they suspense. Like this year's La Niña. In September, the National Weather Service cancelled its months-long lookout for the climate phenomenon—which, as a counterpoint to El Niño, is associated with cooler overall global temperatures. Then, last week, the agency reversed. Its Climate Prediction Center predicted a 70 percent chance of La Niña forming, and folded that prediction into its Winter Weather Outlook. If true, that means the next few months will be warm and dry in the southern half of the US; wet and cool in the north.

If. A La Niña requires months and months and months of persistently cool sea surface temperatures in the equatorial Pacific—and—colluding atmospheric conditions, like stronger than normal trade winds. Unlike the inevitable (and inevitably adorable) first encounter between protagonists in a romcom, there's no guarantee of a geothermal meet cute.

La Niña and El Niño are part of the same climate cycle. And like all things climate, they begin with the sun. Solar energy heats the ocean's surface, creating convection—wind. The Earth's rotation (via the Coriolis effect) makes those winds blow from east to west. Trade winds. They, with help from giant subsurface waves, push the warm surface water westward against the island clusters of Indonesia and Papua New Guinea. "There’s a slow buildup of warmer than average water under the surface in the western tropical Pacific," says Anthony Barnston, chief forecaster for the International Research Institute for Climate and Society at Columbia University. "If you build up enough it will spill over into eastern tropical Pacific." When the spillover happens, the normally cool eastern Pacific becomes unusually warm. The convection slows down, and the trade winds fade, or die. From there, global weather havoc—El Niño.

In all things, balance. La Niña is the yin in the tropical Pacific tajitu. Sometimes, as an El Niño drags on, giant subsurface waves begin to reflect off Indonesia and the other islands. The water there is tepid, drawn in to replace the warm stuff that went east. If the subsurface waves push enough cool water to overcome the balmy El Niño blob, you've got La Niña conditions. (Note: La Niñas don't just form after El Niños, but more on that in a bit.)

"But if the ocean cools off without the atmospheric component, you can't have a La Niña," says Barnston. That means the trade winds have to kick up a notch or so. "That correlates with things like shifts in tropical rainfall patterns, sea level pressure, and upper atmospheric winds." And those drive phenomena like hurricane formation and drought. For the past several months, a crucial area of the central tropical Pacific has been cold enough for La Niña, but the trade winds have been normal.

The people who keep track of La Niñas and El Niños work at NOAA's Climate Prediction Center. The crucial area they watch is a rectangle about the size of the US along the equator between the International Date Line and South America Pacific. It is called the Nino 3.4 region. When the area cools by more than 0.5˚C of the historical average for a three month period, the NOAA climatologists issue a La Niña watch. (They issue an El Niño watch when the same region is 0.5˚C degrees warmer than usual.) And then they watch some more.

"A month ago, a team of us looked at the models, and looked at the numbers, and when averaged together, it didn't seem like the La Niña would hold," says Tom DiLibert, a meteorologist at the Climate Prediction Center. Mostly, this was due to the fact that those nothing special trade winds I mentioned earlier. "Then September happened." The trade winds finally picked up. La Niña watch is back. In order to earn the official La Niña title, the Nino 3.4 region needs to stay 0.5˚C cooler for five consecutive three month periods.

But La Niñas don't always happen after El Niños. Sometimes they just happen. And even the fact that the world just experienced one of the strongest El Niño's on record is no guarantee of a La Niña. It took a full year for a La Niña to form after the super strong 1982-83 El Niño.

Nature, however, doesn't care about the words humans use to describe its atmospheric fluctuations. "If you look at weather patterns in the US, it pretty much conforms to what we'd expect from a weak La Niña," says Barnston. Warm and dry in the south, wet and cool in the north. "Forecasters are calling for effects from this event whether it gets labeled a La Niña or not," he says. Sheesh. Some people just can't wait for the plot to bear out.